The need for better ground and space based telescope resolution has driven the manufacturing of larger diameters of primary mirrors of such telescopes. However, larger diameter primary mirrors result in the primary mirrors having additional weight and manufacturing problems. For example, because large solid mirror blanks weigh more, they require more time to cast and to anneal. The heavier mirror blanks also bend under their own weight, and are more difficult to maneuver in the factory. For space based mirrors, the zero gravity back-out, for testing purposes, is smaller and therefore, simpler and more accurate for light-weight mirrors. Special mounting and supporting is required if accurate testing is to be achieved.
In contrast, light-weight mirror assemblies, fabricated from light-weight mirror blanks, have the advantage of increasing the stiffness-to-weight ratio, and therefore the frequency of the first resonant mode. Light weighted mirror blanks, as it is termed in the industry by those skilled in the art, make the finished mirror assembly more tolerant of spacecraft maneuvers, as well as increasing the mirror's stability. Light-weight mirrors assemblies also result in lighter payloads and lower booster rocket power requirements.
Light-weight mirrors are the desired end product. With respect to FIG. 6, common to all mirrors is a precision surface 650, onto which a reflective material is applied. Prior to application of light weighting techniques, a monolithic material structure, was termed a mirror blank, described as solid mirror blank 450 in FIG. 6a. With improved light weighting techniques, the mass can be reduced by changing the solid mirror blank 450 into a combination of solid material and empty regions. The techniques for generating empty regions or reducing mass while maintaining support for the precise optical surface is what distinguishes the areal density across the various designs. The degree of light weighting, the stiffness to weight ratio, the first mode resonant frequency, and the surface figure profile over a temperature range are just some parameters that distinguish these designs from each other.
Illustrated in FIG. 6b, U.S. Pat. No. 6,598,984 B2, R. Reitz and R. Dahl show that removing material in the non-axial direction is good way to reduce mass. Precise optical surface 650 has remained unchanged; however, empty regions in the once solid block now provide some degree of light weighting.
FIG. 6c shows what could result from a pocket milling process where material is removed from the axial direction by abrasive water jet.
Now, instead of a solid mirror blank 450, FIG. 6d shows how mirror blank 450 has become an assembly of components. We will refer to the aggregate of parts supporting the precise optical surface 650 as the mirror blank assembly 500. In FIG. 6d, the components are comprised of a front face sheet 20, a back face sheet 30, and struts 300.
Once the mirror blank assembly 500 components have been bonded together, the degree of light weighting is a simple ratio of mass to area, typically measured in kg/m2. In 1983, the Hubble Space Telescope primary mirror was designed and constructed to be 180 kg/m2. The Eastman Kodak Company Advanced Mirror System Demonstrator mirror yielded next generation segmented primary mirror prototypes at 15 kg/m2. Future programs have demanded even lower areal densities. This demand has made innovative light weighting techniques a valuable system trade off.
Different inventors have suggested various methods of light weighting mirror blanks. One approach takes a high quality front plate and attaches it to a foam core. For mechanical stiffness, a back plate was usually added to the rear of the foam core. U.S. Pat. No. 4,670,338 issued Jun. 2, 1987 to Alain Clemino and titled “Mirror Foamed Glass Substrate And Method Of Manufacture” discloses a series of foamed blocks glued together and then attached to face sheets. In U.S. Pat. No. 5,208,704 issued May 4, 1993 to Richard R. Zito and titled “Ultralight Mirrors,” a fibrous substrate made from silica and alumina fibers was sealed and subsequently coated with a slurry glaze. The coefficients of thermal expansion (CTE's) were matched to prevent warping. Tatsumasa Nakamura, et al. disclose in U.S. Pat. No. 5,316,564 issued May 31, 1994, and entitled “Method For Preparing The Base Body Of A Reflecting Mirror,” a process to fuse a thin plate to foamed silica using a silicon-rubber curing agent. Nakamura, et al. also disclosed fusing the thin plate using fine glass powder. In U.S. Pat. No. 5,640,282 issued Jun. 17, 1997 to Yoshiaki Ise, et al., and entitled “Base Body of Reflecting Mirror And Method for Preparing the Same,” the inventors disclose attaching a high-quality plate to a porous substrate using silica powders. Claude L. Davis, Jr., et al. (U.S. Pat. No. 6,176,588, issued Jan. 23, 2001, and entitled “Low Cost Light Weight Mirror Blank”) show an optical surface attached to extruded ceramic honeycomb (e.g., Corning's CELCOR®) with room temperature vulcanizing silicon. These approaches all use adhesives that have slightly different CTE's. Also, the bonding materials are hydroscopic and can change dimensions with humidity.
A second approach is described in U.S. Pat. No. 3,713,728, issued Jan. 30, 1973 to Lewis M. Austin, et al.; whereby molten glass is poured around small refractories. The refractories (e.g., Glasrock Foam No. 25) were supported by pins. Later, the refractories were removed. This process resulted in a dimensionally stable mirror blank, however, the degree of light weighting with this process is limited since the walls between the refractories need to the be thick enough to let the molten glass flow between the refractories. FIG. 6c shows the resulting shape for such a process.
In a third approach, a core structure is built up from thin struts and face sheets are attached to the strut structure. U.S. Pat. No. 4,917,934, issued Apr. 17, 1990 to Daniel R. Sempolinski, and entitled “Telescope Mirror Blank and Method of Production” discloses a strut assembly with frit bonding and then bonds the assembly to face plates with frit bondings or tape cast strips. These frit bonds are subject to moisture absorption. Also, struts tend to sag when heated, unless the struts are thick. Thick struts will limit the degree of possible light-weighting. Phillip R. Martineau, in U.S. Pat. No. 6,045,231, issued Apr. 4, 2000, and entitled “Open Core Light-Weight Telescope Mirror And Method of Manufacture” disclosed front and back plates fused to a strut structure by fusing the plates at the softening point. The strut structure is open to the outside diameter, eliminating the need for vent holes. Concerns remain that this design suffers from stability problems especially when the optic is mounted in a trunion or tip/tilt mount. Additionally, strut thickness will limited the degree of light-weighting. FIG. 6d shows what a strut design would look like, independent of the bonding method.
The Hextek Company has successfully made mirrors using their GAS-FUSION® process. In this process, borosilicate glass tubes are pressurized while the tubes are heated between face sheets. The tubes are pressed into a hexagonal close-pack geometry. The temperature is reduced and the pressure is reduced. The result is an 85% light-weight core. While this process yields a structurally sound blank, the industry is now demanding still lighter mirrors. The degree of light-weight is limited by the cells supplying enough structural support after heating and before inflating. Cells too thin will sag after heating.
Russian Patent No. 739458 from Derevensky, et. al. shows closed tubes with spherical bulges. The inventors disclose arranging the tubes such that the spherical regions are in a close-packed orientation, however, the tube arrangement is not maximally dense. The parts are fabricated from sealed tubes. Regions along each tube are heated and blown. Each tube needs to be a custom length and while there may be sets of equal lengths, tubes cannot be fabricated until the overall mirror blank dimensions are known.
Located on the Internet at www.kodak.com, Eastman Kodak Company combined the core structure approach with a low temperature fusion (LTF) process to make several mirrors. The core structure is cut from a solid blank using an abrasive water jet (AWJ) tool. The face sheets are fused to the polished core structure and a back plate is added. However, the LTF process may still be improved upon to reduce manufacturing time and process costs. This design is also a strut construction depicted in FIG. 6d. 
References to corrugated glass can be found in the public domain. For example Joel Berman Glass Studios Ltd., Vancouver, British Columbia, Canada shows a one-dimensional corrugation in either the long or short direction. The C&P Lighting Company, Ltd, Bangkok, Thailand describes a lighting fixture for outdoor lighting that features a “corrugated glass dome.” The Visa Lighting Company of Milwaukee, Wis. carries the Easel line of light diffusers with the “corrugated glass diffuser” option.
In U.S. Pat. No. 3,112,184 R. Hollenbach generated a corrugated carrier with a crimping tool. Later he coated this corrugated structure with ceramic material. Once coated the carrier and unfired material was passed through a furnace. The resulting structure was bonded and cured. The inventor suggests inorganic carriers such as aluminum foil, tea bag paper, nylon cloth, rayon cloth, and polyethylene film. The ceramic materials include the glasses, such as boro-silicates, soda-lime-silicates, lead-silicates, alumino-silicates, etc. The structures resulting from the light-weight ceramic shapes show a one-dimensional corrugation but he mentions more “complex repeating patterns.” Hollenbach discloses using corrugated structures for protecting high-speed projectiles from heat and friction, thereby forming a thermo-barrier. Additionally, Hollenbach teaches stacking the corrugated structures.
In U.S. Pat. No. 3,272,686 by the same company, the honeycomb structures are assembled with flux that match the coefficient of thermal expansion (CTE). The bond is commonly called a frit and is subject to moisture absorption and slow shape change. Again, a corrugated structure is used for insulation and protection against thermal heat.
Many techniques use an assembly of components to build a structure from small thin parts and then bond a face sheet to these components. The face sheet subsequently becomes the precise optical surface. FIG. 6d shows how struts 300, form grill-like support structure for front face sheet 20. Several structures, such as theses, have been mentioned as prior art.
In these aforementioned mirror blank fabrication processes, a supplier requires custom tooling and significant time to build the mirror blank to specification. The costs for tooling, material, and process steps can be prohibitive. Therefore, there exists a need for a method of construction for precise mirror blanks that does not incur such drawbacks and adequately supports the precise optical surface 650. The present invention reduces the fabrication time from tens of months to tens of hours. The areal densities from this process are less than what current technology has recently produced. The costs for this new process are significantly less than current methods. As a result, the risks associated with finishing the mirror blank assembly, have been greatly reduced since fewer resources were required to produce the mirror blank assembly.